Different levels of efficiency, from vehicle miles per hour to vehicles per hour per lane, at different acceptable accident rates.
"A vehicle's road space requirements increase with speed, because drivers must leave more _shy_distance_ between their vehicle and other objects on or beside the roadway. Traffic flow (the number of vehicles that can travel on a road over a particular time period) tends to be maximized at 30-55 mph on highways with no intersections, and at even lower speeds on arterials with signalized intersections."
30-55? That's really defining _it_ for us ;-)
I remembered the "shy distance" from one of the magazines, years ago. I thought the peak efficiency was 45mph on the freeway.
I agree with your boredom factor. A freeway designed for 80 should be boring at 55.
Well, I just did a couple of tests this morning on my 3.8 liter Pontiac Trans Sport. I ran each test 3 times, on the same stretch of road, which is about a
15 degree incline. I had a roughly 4mph tail wind.
I ran the route at 30, 50, and 74 mph, with the van in overdrive for each run. AIR consumption as measured by the Mass Airflow Meter was
1.56kg/minute at 30mph, 1.98 kg/minute at 50mph, and 2.2kg/minute at
75mph.
At 30mph, which is 1/2 mile/minute, that is 3.12kg/mile or 6.86 lb per mile. At 50mph, which is 5/6 (.83) mile/minute, that is 2.38 kg/mile or 5.23 lb/mile At 75 mph, which is 1.23 miles/minute, that is 1.78 kg/mile or 3.92 lb/mile
At 14:1 afr by weight, that is .0714X6.86=0..499 lb of fuel per mile at 30mph. It is .0714*5.23=.373 lb per mile at 50mph, and .0714X3.92=.2798 lb of fuel per mile at 75mph. Gasoline weighs, lets say, 6 lb per (American) gallon, or 1 lb of gas is .166 gallon.
So this car burned, theoretically,.166X..499=.082 gallons per mile at
30mph, and .166X.373= .062 gallons per mile at 50mph. It burned .166X.2798=.046 gallons per mile at 75MPH.
That is 12.2MPG, more or less, at 30mph, steady state. That is 16.13 MPG at 50MPH steady state, and 21.7MPG at 75MPH steady state.
This is more or less in line with my experience driving on the road, where I can get 700km, or 434 miles on a tank in steady 75mph highway cruising ( which is 21.7MPG, and only 300km or so (186 miles) around town, which is 9.3MPG, and 435-450km on a tank at 50-55mph on local highways. 450km is 280 miles, on 20 US gallons, is 14MPG
Granted, the town mileage is FAR from steady state, so my actual mileage is worse than the theoretical mileage in my test. (only 9.3 instead of 12.2. Knocking around on the back highways at roughly 55 Mph is also not steady state, but I'm only getting about 2mpg less than the theoretical. On the highway, I don't run 75 all the time either - I generally drive
115Kph, and occaisionally as fast as 130 - but to get 700km per tank I generally run 115-125kph, and the average and estimated from the test come out right on the money.
On one tank of gas going to Lakeland Florida one April I went 356 miles, or 574km on 14 US gallons - which is, I believe the best I've done - 25.4 miles per US gallon (31MPG canadian). Average for the entire trip down was better than 22MPG american on a 1700 mile trip.
On the way back, though ice, rain, snow and high winds, we averaged little better than 10mpg. We also had a failed O2 sensor on the way home, making the engine think it was running lean all the time, which made it run PIG RICH.
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Just to add to this, the total frontal area of this van is just under
25 sq ft, and the bublished cD I believe is .34 Tractive force requirement is calculated as Da=cDXArew in Sq Ft X (mph)^2/391
So, at 30MPH,.34X25X(30X30)=7650/391=19.56 lbs. At 50MPH, it is .34X25X(50X50)=21250/391=54.34 lbs. At 75MPH it is .34X25X(75X75)=47812/391=122.3 lbs
On a 20% grade (just under 12 degrees inclination) it requires.196* vehiclw weight to climb the hill. With a 3600 lb curb weight, that adds 705 lbs of required thrust. Carrying 200 lbs of "stuff" in the van adds 39 lbs of thrust requirement on a 20% grade, which is more than the aerodynamic drag at 30MPH. 400 lbs of passengers/luggage requires more extra power than the difference in speed from 50 to
75MPH. This is totally disregarding the increase in ROLLING resistance due to added weight. Rolling resistance of a radial tire does not change much with speed,(from 0-30mph, virtually the same, then starts to rize - if .012 at 0-20mph, it gets up to about .013 at 60, and .0137 at 70.) but does change with tire inflation, temperature and load. At speed your tires are warmed up, and the pressure is roughly 5PSI higher than at low speed. For every 20% of the rated load on the rire, the rolling resistance co-efficient goes up about .010. Warming up the tire can reduce the rolling co-efficient by 50% (after about 20 miles of driving) and tire pressures. Dropping tire pressure to half the rated pressure DOUBLES the rolling resistance, and increasing beyond recommended pressure by 50% only lowers the resistance about 20%. Having steering geometry out of spec can cost you as much as underinflated tires.
I got all of this information in manuals from Paul Shipps at 3E vehicles to calculate power requirements for my electric car when I built it over 20 years ago.
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Well, it's only my observation but what about this...
Before my TBirds, I drove a 93 Probe SE 4 cyl 5 speed. I kept, then as now, good track of my fuel mileage. Because the car was fun to drive and relatively underpowered, I had my foot to the floor (WOT) a LOT... and got 26.5 MPG; 1/3 suburb, 2/3 Highway.
One tank, I decided to see what kind of mileage I could get driving like the engine was gonna blow at any minute... So I accelerated at about half-throttle, and drove 60-65 on the freeway (in the semi-truck lane), as opposed to 72-77 mph.
I expected to get about 30 mpg... but only got 27.5, so I kept it up through the next tank, with teeth gritted, and got exactly the same 27.5.
I've done the same with each of my TBirds, and can only gain 1 mpg on them, as well.. from 22.5 to 23.5.. once 24.
It's not worth it to me to pass up a minor grin, just to gain 1 mpg. And when I drove my SVO, I have NO idea WHAT mileage I might have gotten rather than 21-22 mpg... because THAT was BIG GRINS!
I wouldn't knock off close to 10mph to save 1mpg, either.
I think it was wise of you, however, to conduct the experiment.
I've got some experimental evidence that suggests close to a 4mpg drop in fuel economy going from 55-60 up to 70-75 and that's tempting but still probably not worth it. Dropping back from 70-75 to 55-60 would cost me an extra hotel night and extra restaurant meals on a lot of our trips. Fuel's cheaper. Our usual annual 1400 mile (each way) trip would take an extra six hours or so (each way) and we'd save something like $25 on fuel. That's about $4/per hour savings before the extra hotel night and extra McMeals take it away again. Not worth it.
It might be worth it to me to see if there's a some sort of knee in the curve where mpg really falls off and try to hit that speed. There's still a lot of variables. We don't have a mileage computer on board, so I'd have to do some consistent 200+ mile runs at varying speeds and fill the tank consistently (same pump, for a start!) a few times to get a feel for fuel economy. That would be really inconvenient. Then, temperature and prevailing winds would have to be considered. Since I don't have any kind of routine 200-mile trips, this experiment would be difficult to arrange.
You were doing 74mph on a 15 deg (27%) incline???? In a car??? ... But heck, what do I know about the Pontiac Trans Sport? Perhaps it can reach 74mph in first gear...
Considering all your previous posts defending CR, your response doesn't surprise me. In fact if I have any pre-conceived notions at all it's that you're an idiot who believes everything Consumer Reports has ever written.
I don't necessarily believe everything CR has written. They just have way more credibility than you. They test lots of things, appear to learn from their testing and, for the most part, document what they're testing and how.
When I make a major purchase, I'm going to check and see what they have to say and probably check other published sources, too. It's way more effective than reading the internet postings of some unknown quantity called "Bob."
If you don't like their numbers, go borrow cars like that and run your own experiment. Copy their method or create and document your own.
Given the shape of the various speed/economy curves that Oak Ridge published, I'm not at all surprised to find a result that looks linear when expressed to the nearest 1 mpg.
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